The present invention relates to a process aid for heat cured silicone rubber composition and more particularly, the present invention relates to process aids in which the basic ingredient is a cyclictrisiloxane for use with heat cured silicone rubber compositions.
Heat cured silicone rubber compositions are well known in the silicone art. Such heat cured silicone rubber compositions have as the basic ingredient a linear diorganopolysiloxane polymer having a viscosity anywhere from 1,000,000 to 2000,000,000 centipoise at 25.degree. C., a filler which may be extending or reinforcing filler: examples of reinforcing fillers being fumed silica and precipitated silica. such compositions also include a catalyst which is usually a peroxide curing catalyst. Such a composition can also include other ingredients such as self-bonding additives, compression set additives, heat aging additives, post cure additive, etc. In the fabrication of such compositions it is common to mix the filler and the diorganopolysiloxane polymer in the dough mixer and then add all the other necessary ingredients except for the peroxide curing catalyst. The composition is then cleaned by straining it through a screen, adding a peroxide catalyst to it and then fabricating or extruding it into various desired parts. Such prefabricated compositions are then shipped from the silicone manufacturing facilities to the various customers who take the composition and perform additional finishing operations on the fabricated part and then heat it at elevated temperatures that are above 100.degree. C. to form a silicone elastomer. In other types of compositions shipped from silicone manufacturing facilities the composition with a filler and the other additives, but without any catalyst, is shipped. The purchaser of the composition takes it and adds to it the desired amount of peroxide curing catalyst then fabricates it or extrudes it into a desired part after which the composition is heated at temperatures above 100.degree. C. to produce the desired silicone elastomeric part.
It was early noticed in the fabrication of cured silicone rubber compositions that the prefabricated parts or even slabs of unfabricated uncured silicone rubber compositions would structure with aging it even relatively short periods of time such as a month or less. That is the composition which would set in the position it was stored and the shape of the uncured silicone rubber composition could be changed only with difficulty.
Accordingly, such compositions would set in what is known in the silicone art as crepe hardening. Accordingly, because of such crepe hardening, uncured silicone compositions which had been stored for periods of a month or more would be very difficult to mill on mills to incorporate catalyst into them or it would be very difficult to extrude or to fabricate into the desired part. Because of such crepe hardening, uncured silicone rubber compositions of the past were difficult to process. It was desired to have uncured heat cured silicone rubber compositions that would not structure or set for a period of a year or more after manufacture and which would have the same amount of swelling in an extrusion die for a period of a year or more. In addition, it was desired that the uncured heat curable silicone rubber composition would not structure for a period of a year or more after manufacture so that it did not have to be intentionally remilled to make it flowable before it could be fabricated to the desired part. Accordingly, to correct this problem there was developed as what is known in the silicone art as process aids. Such process aids which for the most part were silicone polymers, are incorporated into the diorganopolysiloxane polymer of heat cured silicone rubber compositions along with a filler. As the result of the use of such process aids and the more preferable ones, it was found that structuring could be substantially reduced if not eliminated. Examples of such desirable process aids are for instance those to be found in the Martellock U.S. Pat. Nos. 3,464,945 Fekete 2,954,357 and Konkol, et al, 2,890,188. Although such process aids work well in the production of heat cured silicone rubber compositions to prevent structuring; nevertheless, they were expensive to produce and they were used in quantities of anywhere from 1 to 25 parts per hundred parts by weight of diorganopolysiloxane polymer. This necessarily increases the cost of the heat cured silicone rubber composition. In addition, the processes for producing such process aid such as that disclosed above, were somewhat difficult to carry out requiring a certain amount of planning, equipment and manpower. Accordingly, it was highly desirable to develop an inexpensive process aid, and specifically, one that would utilize known by-products that are produced in silicone manufacturing plants. It should also be noted that such prior art processes for producing process aids as disclosed in the foregoing patents also involved the use at times of solvents, which added to environmental problems in their disposal after the desired process aid had been produced. Accordingly, it was also highly desirable to utilize as a process aid silicone compounds that were not found from undesirable reactants or solvents or created by-products that had to be disposed of properly in view of environmental restrictions.
One approach to this problem was the use of trifluoracetic acid as disclosed in Karstedt, et al U.S. Pat. No. 3,671,546. However, as noted in that disclosure trifluoracetic acid was added primarily to prevent the filler from clumping or balling up when it was mixed into the base diorganopolysiloxane polymer. It was disclosed to be utilized as a supplement to traditional process aids that were used with heat cured silicone rubber composition. Accordingly, Karstedt, et al Patent had not found a substitute for traditional process aids in the production of heat cured silicone compositions.
Accordingly, it was high desirable to produce a process aid that could be produced in situ in the mixing apparatus for the heat curable silicone rubber composition when the diorganopolysiloxane polymer was mixed with the filler. It was also highly desirable to have a process aid that not only prevented structuring in heat curable silicone rubber compositions, but also prevented clumping and balling up of filler when it was mixed in diorganopolysiloxane polymer in the dough mixer.
Accordingly, it is one object of the present invention to provide for an inexpensive process aid for heat curable silicone rubber composition.
It is another object of the present invention to provide process aid which can be prepared in situ in the mixing equipment for heat curable silicone rubber compositions.
It is an additional object of the present invention to produce a process aid for heat curable silicone rubber utilizing as a reactant a compound which is readily available by-product at silicone manufacturing facilities.
It is yet an additional object of the present invention to provide for a process aid for heat curable silicone rubber composition which does not necessitate special equipment or process procedures to prepare.
It is still another object of the present invention to provide a simple process for producing process aids for heat curable silicone rubber compositions. These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.